1. Field of the Invention
This invention generally relates to an apparatus for and a method of electro-optically reading symbols, and, more particularly, to a laser beam scanning system capable of detecting a scanning range of a bar code symbol and adjusting the focus of a scanning laser beam based on the scanning range.
2. Description of the Related Art
Optical readers and optical scanning systems for scanning and reading indicia, such as bar code symbols, appearing on a label or on the surface of an article, are commonly used. A bar code symbol is a coded pattern of graphic indicia comprised of a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light-reflecting characteristics. The scanning systems electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumerical characters that describe the article or some characteristic thereof.
Generally, scanning systems include a light-emitting source, such as a gas or semiconductor laser, to illuminate the symbol with a laser beam that may be optically adjusted by an optical focus assembly, to form a beam spot of a certain size at the target distance. It is preferred that the cross section of the beam spot at the target distance be approximately the same as the minimum width between regions of different light reflectivity, i.e., the bars and spaces of the symbol.
A lens or similar optical components direct the laser light beam along a light path toward a target that includes the bar code or other symbol. A photosensor or photodetector detects the light reflected or scattered from the target bar code symbol. The photodetector is positioned in the scanner along an optical path so that it has a field of view which ensures the capture of a portion of the light reflected or scattered off the targeted bar code symbol. This captured portion of reflected light is subsequently detected and converted into an electrical signal. Electronic circuitry or software decode the electrical signal into a digital representation of the data represented by the symbol that has been scanned. For example, the analog electrical signal outputted by the photodetector may be converted into a pulse width modulated digital signal, with the widths corresponding to the physical widths of the bars and spaces. Such a digitized signal is then decoded based upon the specific symbology used by the symbol into a binary representation of the data encoded in the symbol, and subsequently to the corresponding alphanumerical characters.
As previously stated, laser beam scanning systems employ a laser light beam that is directed by a lens along a light path towards a target that includes the bar code symbol. The moving-beam scanner operates by repetitively scanning the light beam in a line or series of lines across the coded symbol by motion of a scanning component, such as the light source itself, or a mirror disposed in the path of the light beam. The scanning component may either sweep the beam spot across the coded symbol and trace a scan line or pattern across the symbol, or scan the field of view of the scanner, or do both.
Scanners establish scanning distance or range finding by incorporating laser focus control techniques. These techniques, which include for example, lens changing mechanisms, slidably driven lenses, piezo-electric focusing elements, and the like, typically deploy a range information bearing signal to indicate the degree of focus adjustments.
However, without a method of intelligently making a decision as to when to adjust the focus, the scanning range response becomes sluggish and the system becomes cumbersome to use. For example, a simple method to detect scanning range would be to monitor the received signal strength of the light signal reflected from the targeted bar code symbol. In theory, because the power of the received light beam is inversely proportional to the square of the distance from the target, the further away the targeted symbol, the longer the scanning distance, and the weaker the received signal strength. Similarly, the closer the targeted symbol to the light-emitting source, the shorter the scanning distance, and the stronger the received signal strength. The level of received signal strength is related to the scanning range and may be used to adjust the focus at any given instant. This method will only work, however, if the prevailing conditions are pristine enough to ensure both consistent bar code quality and constant ambient light levels. If bar codes are printed on different substrates or contrast levels, or if ambient light levels vary, the received signal would be influenced by such variations, resulting in the impairment of the scanning range detection.